There are two mechanisms available for transporting gases between the ambient atmosphere and the pulmonary blood-gas interface: convection and gas-phase diffusion. When transport is limited to convection, the convection-perfusion ratio (VA/Q) determines the O2 and CO2 concentrations as well as the gas exchange ratio, R, at the blood-gas interface in the lung. When transport occurs by gas-phase diffusion (expressed as a conductance, G), then the O2 and CO2 concentrations at the blood-gas boundary will depend upon particular G/Q ratios. The long-term objective is to provide a quantitative description of the interaction between gas-phase diffusion and blood flow and the role which this interaction plays in gas exchange. Whereas the influence of VA/Q on gas exchange has been studied in depth, little work has been done on G/Q distribution and its possible effect on gas exchange. Since measurements of conductance at the pulmonary blood-gas interface are not feasible at present, the proposed research will be done in the hen's egg, an ideal in vivo model for studying O2 and CO2 transport by gas-phase diffusion because these gases are supplied to (or removed from) the blood-gas interface by diffusion through gas-filled pores in the eggshell. The specific aims are to measure, in different regions of the eggshell, (a) the diffusive conductance, G, of the shell to gases; (b) the chorioallantoic blood flow, Q; and (c) PO2 and PCO2 in the air spaces of the developing hen's egg. From these measurements quantitative relationships between G/Q ratios, resulting O2 and CO2 tensions, and the gas exchange ratio, R, will be developed. Diffusive conductance will be measured by following regional water vapor loss under known conditions of water vapor pressure difference and chorioallantoic blood flow by use of radioactive microspheres. Regional PO2 and PCO2 measurements will be made by circulating air space gas in a closed circuit past O2 and CO2 electrodes.